The transfer of very small, dry toner particles from a photoconductor or other image member to a receiving sheet is extremely challenging. Conventional dry toner transfer is accomplished using an electrostatic field. However, studies on the forces which move small particles indicate that, as the particle becomes smaller, the effect of the electrostatic field is less on a particle compared to the effect of ordinary adhesive forces. This has made conventional transfer using an electrostatic field more difficult the smaller the particle.
Because it has been considered not possible to electrostatically transfer particles as small as 3.5 microns in diameter with reasonable efficiency, other transfer means have been investigated. For example, particles as small as 3.5 microns in diameter are transferred by heating them to a temperature which causes them to sinter at least where they touch the surface to which they are to be transferred and to each other; see, for example, U.S. Pat. No. 5,089,363, Rimai et al.
The use of intermediates in electrostatic imaging has also been known for many years and has been used commercially in recent years. Typically, the toner image is formed on a primary image member, for example, a photoconductive member. The image is transferred from the primary image member to an intermediate image member and then from the intermediate to a receiving sheet. Although this approach has some advantages in single-color imaging, it has its greatest advantages in multicolor imaging where the intermediate can be used to receive a number of single-color images in registration to form the multicolor image.
U.S. Pat. No. 5,084,735 discloses the use of particular materials which enhance intermediate transfer of dry toner images made up of particles having a mean particle size of less than 15 microns, including examples in which toners having a mean particle size of 7 microns were effectively transferred to 20 pound bond paper. Especially spectacular results were achieved with an intermediate roller having a relatively soft polyurethane base having a Youngs modulus of about 10.sup.6 Newtons/m.sup.2 with a very thin skin or overcoat of a harder material having a Youngs modulus of about 10.sup.8 Newtons/m.sup.2. Although skins of 1 mil were effective, best results were obtained when the skin was less than 10 microns in thickness.
U.S. patent application Ser. No. 843,587, filed Feb. 28, 1992 in the name of McCabe, describes a toner comprising very small particles of pigmented thermoplastic resin having on their surfaces a coating of small particles which are applied to an aqueous dispersion in a uniform distribution and are strongly adhered to the toner particles. This application discloses extremely small particles of colloidal silica, aluminum oxide or a latex polymer or copolymer of a size less than 0.2 microns which, when properly adhering to the toner particles, can assist in the transfer of such particles.
U.S. patent application Ser. Nos. 843,664 and 843,666, filed Feb. 28, 1992 to Aslam et al, disclose examples of the transfer of toners prepared by the methods disclosed in the McCabe application, which toners have a mean particle diameter of 3.5 microns. The transfer is accomplished by a combination of heat and electrostatic field from a photoconductive surface to a conductive intermediate, for example, a metal sheet. The toner is then transferred to a receiving sheet utilizing heat and fused in the same step. Transfers as high or higher than 99% efficiency were obtained with this process to high quality receiving paper.
U.S. Pat. No. 5,084,735 and U.S. patent application Ser. Nos. 843,587; 843,664 and 843,666 are all incorporated by reference in this application.